1. Field of the Invention
The present invention relates generally to an optical transmitting/receiving apparatus in communication systems. More particularly, the present invention relates to an optical transmitting/receiving apparatus in communication systems in which signals are bidirectionally transmitted in a time divisional manner by way of a single transmission medium.
2. Description of Related Art
Bidirectional transmission by a single optical fiber is very useful because of reduced requirement for the optical fiber. In this type of transmission system, it is necessary to provide an optical transmitting/receiving apparatus having both transmitting and receiving functions at terminals of the optical communication system.
The construction of a conventional optical transmitting/receiving apparatus in communication systems includes a separately arranged semiconductor laser to transmit signals into an optical fiber and a photodiode to receive signals from an optical fiber. Both the semiconductor laser and photodiode are coupled to the same optical fiber with an optical directional coupler. The semiconductor laser and the photodiode are connected to a laser driving circuit and a current/voltage converting circuit, respectively.
Transmission data input from one side of a terminal is converted into current signals for driving the semiconductor laser in the laser driving circuit. The semiconductor laser functions as an electric/optical converting element and outputs light signals having a level that is set in response to the current signals. These light signals pass through the optical directional coupler and then are output to the optical fiber.
On the other hand, light signals input from the optical fiber to the optical transmitting/receiving apparatus are coupled to the photodiode by the optical directional coupler. Changes in current are produced by the photodiode in response to intensity changes of the input light signals. Further, these current changes are converted into voltage changes by the current/voltage converting circuit. Then, these are output to a terminal as the received data.
In the above conventional optical transmitting/receiving apparatus in bidirectional communication systems, both the semiconductor laser and photodiode are required to separately output transmission signals and collect received signals. In addition, the optical directional coupler is necessary to conduct the light from the semiconductor laser into the optical fiber and to conduct the light from the optical fiber to the photodiode.
Subsequent optical communication systems in particular require more compact terminals. Discretely arranged, however, these optical elements prevent the terminals from being reduced in size. Also, complex manufacturing stages of this optical transmitting/receiving circuit are involved.
On the contrary, in half duplex transmission systems, so-called "ping-pong" communication systems, the semiconductor laser and photodiode are not always necessary at the same time because simultaneous operation is not required. In such half duplex transmission systems, more compact terminals are also required.